Three-Dimensional Imaging in Orthodontics

Kara J. McCulloch, DMD
Article by
Mercer Island Orthodontist

Dental and facial x-rays have long played an indispensable role in orthodontic diagnosis, treatment planning, and documentation. With the advent of three-dimensional (3-D) imaging, concerns about radiation exposure have dictated the need for new prescribing protocols. In this newsletter we present some basic information about cone-beam computerized tomography (CBCT) and the circumstances under which we have adopted its use.

In this age of consumer awareness, patients and parents are becoming ever more inquisitive about recommendations made by health care professionals. We have found, as we are sure you have too, that questions about radiation exposure have become increasingly common. If it ever was acceptable to describe dental x-rays as “safe, necessary and routine” it certainly is no longer the case. As imaging technology becomes more sophisticated we are increasingly challenged to elicit the maximum amount of information with the minimum of patient radiation exposure.


What is CBCT?
CBCT is a process in which radiographic images are captured digitally and then undergo software manipulation that provides constructed three-dimensional images. These images can, in turn, be viewed in high resolution with an infinite range of spatial orientations and “slice-thicknesses”. At Orthodontic Associates of Mercer Island we prescribe CBCT scans using the iCAT (Imaging Sciences International) hardware and software systems available through Northwest Radiography’s imaging laboratories.
Under what circumstances do we prescribe CBCT scans?


We subscribe to the American Dental Association’s “ALARA” (As Low As Reasonably Achievable) principal and thus eschew a “cook-book” approach to ordering x-rays. We consider 3-D imaging to be appropriate only under those circumstances in which conventional imaging will not provide sufficiently detailed information to serve the individual patient’s best interest. Examples of specific clinical situations to which CBCT scans are applicable are:


Ectopic teeth, particularly maxillary canines – Localization of ectopic teeth has conventionally been accomplished by interpreting the parallax-effect seen on multiple periapical radiographs taken at different angles (SLOB). This approach serves well to distinguish between lingual and buccal position of the ectopic tooth. However, it often proves inadequate to elucidate other important elements such as proximity of the supernumerary tooth to roots of other teeth and resorptive damage to contiguous tooth surfaces. In addition to allowing definition of such diagnostic parameters, 3-D manipulation of CBCT images enables, otherwise unavailable, spatial visualization. This often proves critical in exactly defining traction-attachment placement at the time of surgical exposure and in planning the most effective vectors through which orthodontic traction is to be applied.


Multiple supernumerary teeth – In patients with multiple supernumerary teeth, the decision as to which tooth (teeth) to extract is often made with difficulty when only conventional imaging is available. CBCT scans allow detailed examination of crown and root morphology, spatial relationships, and anticipated eruption pathways. This is particularly important in the event extraction of one or more teeth is indicated prior to emergence since the extraction decision can be made with relative confidence in advance of surgical exposure rather than intra-operatively.


Tempromandibular joint dysplasia – CBCT imaging is indispensable in examining altered tempromandibular joint morphology, in particular since orthodontic treatment planning often intersects with management of juvenile rheumatoid (or idiopathic) arthritis, idiopathic condylysis, post-traumatic joint damage, or other circumstances in which facial development is adversely effected. The ability to examine joint structures in three dimensions and to serially document such processes has increased the scope of our diagnostic and treatment planning capability.


Pathology – The finding of occult pathology (dentigerous/follicular/traumatic-bone cysts, odontogenic neoplasms, giant cell lesions, etc.) on conventional dental radiographs is not uncommon. CBCT scans allow the nature of such lesions to be examined in greater detail for diagnostic purposes and their extent evaluated in three-dimensions prior to any indicated surgical intervention.


Congenital/developmental/traumatic facial deformities – The planning of combined orthodontic and orthognathic surgical treatment for patients requiring such correction is heavily dependent on detailed study of facial skeletal relationships, proportions, and balance in multidimensional space. While in the past this has been accomplished with fair facility employing two-dimensional imaging, the accuracy with which treatment planning can be carried out is much enhanced when CBCT imaging forms the basis for this complex decision-making process.

What is CBCT radiation exposure in comparison to that of other dental imaging?
The National Council on Radiation Protection and Measurement has estimated that the mean effective radiation dose-equivalent from all sources is 3600 µSv. Dental imaging is estimated to contribute approximately 5% of that total mean exposure. At 92.8 µSv CBCT is closer to the range for a standard panelipse radiograph than it is to a conventional full-mouth series. Thus while not insignificant, exposure for 3-D radiography is relatively conservative, particularly when one takes in to account the amount of information captured and the detail with which it can be analyzed.
 

Radiographic Source                                                                                         Effective Dose Delivered (µSv)
All Radiation Sources (National Average)                                                                         3600
Natural (Background) Radiation                                                                                         3000
Manmade Radiation(industrial, medical, dental)                                                              600
Dental Radiation                                                                                                                      200
Full Mouth Series (D-Speed / F-Speed)                                                                       338 / 170.7
22 cm FOV CBCT                                                                                                                     92.8
2-D Panoramic Film                                                                                                           14.2-24.3
2-D Lateral Cephalometric Headfilm                                                                                5.1-5.6
 

It is “standard-of-practice” in orthodontics to use lateral cephalometric radiographs, panelipse radiographs, and selected periapical radiographs for diagnosis and treatment planning. Since this suite of films is obtainable with approximately 50% of the dosage required for a CBCT scan, it is evident that the latter should be reserved for unique situations in which multidimensional analysis has high value. At the rate of evolution of digital technology, however, it is clear that in the not too distant future CBCT will be available at exposure levels and in degrees of resolution equivalent to today’s two–dimensional imaging techniques. As that time approaches we at Orthodontic Associates of Mercer Island will find this powerful tool to have increasing applicability as we strive to provide the highest quality of comprehensive patient care.


Summary-
Cone-beam computerized tomography is a digital imaging process that provides indispensable diagnostic and treatment planning information that can be viewed and analysed in multiple dimensions and at high resolution. The use of CBCT scans, in lieu of two-dimensional imaging, is appropriately limited to those situations in which the greater radiation exposure level is balanced by the additional value of the anatomical information obtained. At Orthodontic Associates of Mercer Island we currently prescribe CBCT imaging only for those patients for whom critical diagnostic and treatment planning decisions cannot be made with confidence in its absence.


References:
1. Bell, J. Executive Director, Northwest Radiography, Inc., Personal Communication.
2. Hatcher, D. “Operational Principals for Cone-Beam Computed Tomography” 2101; JADA, Vol 141, No Suppl 3, 3S-6S.
3. ADA website:  Oral Health Topics : Radiography
4. AAO website
5. NCRP website
6. Kapila, S. et al. The current status of cone beam computed tomography imaging in orthodontics. Dentomaxillofacial radiology (2011) 40, 24-34.

© Copyright R.W. McNeill, DDS, MS, K.J. McCulloch, DMD, MSD